It is, of course, very important that the valves which are used for controlling the various production flows should be intact. However, the valves operate under difficult conditions, both as far as the actual fluids are concerned and as far as the surroundings are concerned. To ensure satisfactory operation, the valves have to be capable of being repaired, and, possibly, non-operating parts of the valves have to be capable of being replaced. Normally, therefore, the valves are designed so as to have their vital parts built into a replaceable insert unit. Upon replacement, the existing non-operating insert unit must first be removed from its valve housing, whereafter a new insert unit is mounted into the valve housing. To ensure satisfactory sealing between the valve housing and the insert unit, relatively high tightening moments are applied to the nut--normally a castellated nut--which holds the valve housing and the insert unit together. When detaching the insert unit, a releasing moment of at least the same magnitude is normally required.
Since a valve tree comprises a plurality of valves whose insert units may need to be replaced, the tool that is to bring about the releasing and the tightening, respectively, of the castellated nut must be capable of being positioned against the valve in question. It is also desirable for this part of the replacement process to be carried out by means of remote operation.
An example of a device for positioning a tool for replacement of insert units is given in the above-mentioned STU publication. The oil company Exxon has in its SPS system a manipulator which is capable of being moved on a rail system secured to the surrounding steel structure of the wellhead. The movement around in the production tree and the associated pipe system takes place by means of a rack. The positioning is remote-controlled and takes place, among other things, by means of TV and video cameras.
Secured to and built into the manipulator are moment producing devices intended for releasing and tightening, respectively, the castellated nut which fixes the insert unit of the valve to the valve housing, as well as for threading in and out the insert unit. The reason for having different moment producing devices for effecting the releasing and the tightening moment, respectively, and for effecting the moment for threading the nut in and out is that the difference between the magnitude of the two moments required is considerable. The high releasing and tightening moment in the Exxon design, which is generated in the above-mentioned manipulator, is brought about by allowing two hydraulically operated piston rods to act against two projections on a rotatable ring which is in engagement with and which surrounds the castellated nut. In the Exxon manipulator, the moment for threading the nut in and out is brought about with the aid of a worm gear which engages external splines on the rotatable ring mentioned.
The problems with the Exxon design and similar designs are manifold. The manipulator including the moment producing devices as well as devices for positioning consist of large and unwieldy structures which may have a weight of some twenty or thirty tons or more. The high weight necessitates that the manipulator is bottom-based and that transportation must take place on some form of rail system. Also the associated rack structure for transportation will therefore require heavy dimensions, the power requirement for the positioning being correspondingly high. Moment producing devices for releasing and tightening, respectively, of the castellated valve nut also have a limited possibility of rotary motions. The fact that two different moment producing devices are needed to release the insert unit must also be considered a less successful solution.
Therefore, for a long time there has been a need of light and easily manageable constructions, improved moment producing devices, etc., to facilitate the process when replacing an insert unit in valves used in SPS systems for oil and gas production.